The invention relates generally to integrated circuits and, more particularly, to structures and techniques for reducing electromagnetic radiation from integrated circuits.
Some types of integrated circuits (ICs) are known to radiate a relatively large amount of electromagnetic energy during periods of operation. For example, microprocessors and other digital processing devices are recognized as a major source of electromagnetic radiation in computer systems. The electromagnetic energy radiated by such devices can interfere with the operation of other devices or circuits in the vicinity of the radiating IC and is therefore undesirable. For this reason, electromagnetic interference (EMI) standards have been developed to place limits on the amount of radiation that ICs can generate during operation.
A number of techniques have been used in the past to reduce the level of radiation emanating from an IC. In one prior approach, for example, a conductive cage structure is formed about the IC using a stamped sheet metal cage member that has a plurality of legs spaced about a periphery thereof. The cage member is placed over the IC package where it is coupled to a cover plate of the package. Each of the legs of the cage member is coupled to a corresponding ground pad on an underlying circuit board. The ground pads of the circuit board are each conductively coupled to a ground plane of the circuit board using, for example, via connections. In this manner, a conductive shield is formed about the IC that will confine most of the electromagnetic energy radiated by the IC within a bounded area surrounding the IC during subsequent operation. A conductive shield such as this is known as a Faraday cage. The spacing between adjacent legs of the cage member is related to the maximum expected radiation frequency of the IC. That is, the legs need to be closer together to contain higher frequencies than they need to be to contain lower frequencies.
As can be appreciated, implementation of the above described approach will typically consume a relatively large amount of real estate on the underlying circuit board. That is, the ground pads to which the legs of the cage member are to be coupled will require a considerable amount of surface space on the circuit board that would otherwise be available to other circuitry. In addition, the above approach will typically require a significant amount of assembly time to, for example, correctly attach the cage member to the IC package with the legs of the cage member properly aligned with and coupled to the ground pads. Furthermore, because the frequencies of ICs are continually increasing, the distance between adjacent legs of the cage member in the above approach will need to be reduced over time. This will require an even greater number of ground pads on the circuit board and even more assembly time to implement. Eventually, a point will be reached where it is not practical to add further legs to the cage member and adequate EMI suppression will not be possible using the above approach.